In recent years, Doherty amplifiers have been proposed as high efficiency amplifiers for communications.
In a Doherty amplifier, a carrier amplifier being biased as class AB or class B and a peak amplifier being biased as class C are connected in parallel.
Division circuitry is connected at a preceding stage of the carrier amplifier and the peak amplifier, which are connected in parallel. The division circuitry splits a signal to be amplified between the carrier amplifier and the peak amplifier.
Further, a power combiner for combining the signals amplified by the carrier amplifier and the peak amplifier is connected at a subsequent stage of the carrier amplifier and the peak amplifier.
Note that the carrier amplifier performs signal amplification continuously, whereas the peak amplifier performs signal amplification only when high power output is required.
A Doherty amplifier disclosed in Patent Literature 1 below comprises division circuitry and an isolation resistor. The division circuitry includes four λ/4 lines having characteristic impedance that depends on a sprit ratio of signal power to a carrier amplifier and a peak amplifier. The isolation resistor has a resistance value that depends on the sprit ratio of the signal power.
The λ/4 line is a distributed constant line having an electrical length that is a quarter wavelength of a signal to be amplified.